qt: Makefile.am: use recursive Makefile

The project started to move away from recursive Makefile a while ago for
performance reasons. Unfortunately, even though tools like libtool can
handle an arbitrary number of files by using @filelists, automake is
subject[^1][^2] to max argument limitation in its `distdir-am` target,
which will try to expand $(DISTFILES) in the bash command line.

[^1]: https://lists.gnu.org/archive/html/bug-automake/2009-07/msg00008.html
[^2]: https://lists.gnu.org/archive/html/automake/2014-05/msg00015.html

This is workarounded in this commit by moving all the files from Qt in a
separate Makefile that will be recursively called by automake, reducing
the size of $(DISTFILES). Hopefully, Make/Automake would have fixed this
or we would have moved completely to Meson when we reach the limit
again.

configure.ac

@@ -4703,6 +4703,7 @@ AC_CONFIG_FILES([
   Makefile
   doc/Makefile
   modules/Makefile
+  modules/gui/qt/Makefile
   m4/Makefile
   po/Makefile.in
   share/Makefile

modules/gui/Makefile.am

@@ -1,3 +1,5 @@
+SUBDIRS += gui/qt
+
 guidir = $(pluginsdir)/gui
 gui_LTLIBRARIES =
 
@@ -21,5 +23,4 @@ EXTRA_LTLIBRARIES += libminimal_macosx_plugin.la
 gui_LTLIBRARIES += $(LTLIBminimal_macosx)
 
 include gui/macosx/Makefile.am
-include gui/qt/Makefile.am
 include gui/skins2/Makefile.am